During alcoholic fermentation, sugar, particularly glucose, is separated into ethanol and carbon dioxide in the presence of yeast cells which contain the enzyme complex zymase. Glucose is produced by enzymatic splitting of maltose, which is itself formed during the hydrolytic, enzymatic splitting of starch or is developed during the manufacture of sugar. In addition to ethanol, the ethanol solutions developed during alcoholic fermentation contain soluble and insoluble components of vegetable cells and builders, yeast cells, starch and fractions of starch, various sugars, salts and water. The ethanol content of the solutions obtained during alcoholic fermentation is usually about 6 weight percent, since a higher ethanol concentration would reduce the reaction rate of the alcoholic fermentation too much.
Ethanol can be separated from the solutions obtained during alcoholic fermentation by means of distillation processes, and can be concentrated to a purity of 99.9 weight percent. However, distillation processes have the drawback that they require a large amount of energy. Distillation of up to an ethanol concentration of only 95 weight percent requires about 8000 kJ/kg of distillate, while the caloric value of ethanol is 26,780 kJ/kg. For concentrating beyond the azeotropic point, which for 1 bar lies at 95.58 weight percent ethanol, a considerable additional amount of energy is required. The high energy cost of ethanol separation by distillation is an economic impediment for using ethanol produced by alcoholic fermentation as an engine fuel.
To avoid the high energy costs for separating ethanol by distillation, ethanol can be separated from solution by means of extraction with a solvent. German laid-open patent application No. 3,033,729 discloses a method for separating ethanol from aqueous solution, wherein the ethanol containing solution is extracted with CO.sub.2, C.sub.2 H.sub.4 or C.sub.2 H.sub.6 in the form of liquids or supercritical gases. If CO.sub.2 is used as the extracting agent, the extraction can take place at 30 to 150 atmospheres and at 0.degree. to 150.degree. C. In this known process the pressure of the ethanol containing extract phase is reduced and the ethanol is separated from the extraction agent by distillation. A considerable amount of energy and a large amount of apparatus is required for processing the ethanol containing extract phase.